CO2 recovery system and CO2 recovery method

ABSTRACT

A CO 2  recovery system includes an absorber  2  and a regenerator  3 . The absorber  2  includes a CO 2  absorbing section  21  and a water-washing section  22 . The CO 2  absorbing section  21  allows flue gas  101  to come into contact with a basic amine compound absorbent  103  so that the basic amine compound absorbent  103  absorbs CO 2  in the flue gas  101 . The water-washing section  22  allows the decarbonated flue gas  101 A in which the amount of CO 2  has been reduced in the CO 2  absorbing section  21  to come into contact with circulating wash water  104  and to be washed with the wash water  104  so that the amounts of the basic amine compounds entrained in the decarbonated flue gas  101 A are reduced. The regenerator  3  releases CO 2  from the basic amine compound absorbent  103  the CO 2  absorbed therein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of U.S. application Ser. No. 12/901,051 filed Oct. 8, 2010, now U.S. Pat. No. 8,398,758 issued Mar. 19, 2013.

TECHNICAL FIELD

The present invention relates to a CO₂ recovery system and a CO₂ recovery method for reducing the concentrations of residual basic amine compounds emitted together with decarbonated flue gas in which the amount of CO₂ has been reduced by contact with an absorbent.

BACKGROUND ART

The greenhouse effect due to CO₂ has been pointed out as one of the causes of global warming, and there is an urgent need to take global measures against the greenhouse effect to protect the global environment. The sources of CO₂ can be found in various fields of human activities in which fossil fuels are burnt, and there is an increasing tendency to tighten CO₂ emission regulations. Accordingly, extensive studies have been conducted on CO₂ recovery methods applicable to power generation facilities, such as thermal power plants, which use a large amount of fossil fuels. In these methods, flue gas from a boiler is brought into contact with an amine-based absorbent such as an aqueous solution of an amine compound to collect and recover CO₂ in the flue gas.

When such an absorbent is used to recover CO₂ from flue gas, the amine compound is entrained in the decarbonated flue gas in which the amount of CO₂ has been reduced. Therefore, to prevent air pollution by the amine compound, the amount of the amine compound emitted together with the decarbonated flue gas must be reduced.

PTL 1 discloses a conventional amine recovery system. This system includes a plurality of stages of water-washing units for collecting an amine compound entrained in decarbonated flue gas. In each water-washing unit stage, wash water is brought into gas-liquid contact with the decarbonated flue gas in which the amount of CO₂ has been reduced by gas-liquid contact with an absorbent that absorbs CO₂. The amine entrained in the decarbonated flue gas is collected sequentially in the plurality of stages of the water-washing units. As the wash water used in PTL 1, condensed water is used which is produced by condensing and separating water contained in CO₂ during a process of regenerating the amine-based absorbent by removing CO₂ from the CO₂-absorbed amine-based absorbent.

PTL 2 discloses a conventional decarbonation system including: a cooling unit for cooling decarbonated flue gas in which the amount of CO₂ has been reduced by gas-liquid contact with an absorbent; and a contact unit in which condensed water condensed in the cooling unit is brought into countercurrent contact with the decarbonated flue gas. PTL 2 discloses another decarbonation system that includes a water-washing unit for collecting the amine compound entrained in decarbonated flue gas by bringing wash water into gas-liquid contact with the decarbonated flue gas in which the amount of CO₂ has been reduced by gas-liquid contact with an absorbent. The wash water used is condensed-water condensed in a cooling tower for cooling the flue gas from which CO₂ has not been recovered.

CITATION LIST Patent Literature

-   [PTL 1] JP 2002-126439A -   [PTL 2] JP H08-80421A

SUMMARY OF INVENTION Technical Problem

In recent years, from the viewpoint of environmental protection, there is a demand to further reduce the concentrations of residual absorbent components emitted together with decarbonated flue gas. It is expected in the future that a CO₂ recovery system is applied to flue gas from, for example, a thermal power plant in which the flow rate of processed gas is high. In this case, the emission amounts of residual absorbent components emitted together with the decarbonated flue gas tend to increase because the emission amount of the flue gas is high. Therefore, the concentrations of the emitted absorbent components must be further reduced.

The present invention solves the foregoing problems, and it is an object of the invention to provide a CO₂ recovery system and a CO₂ recovery method that can further reduce the concentration of residual basic amine compounds emitted together with decarbonated flue gas.

Solution to Problem

According to an aspect of the present invention, a CO₂ recovery system includes: an absorber including a CO₂ absorbing section and a water-washing section, the CO₂ absorbing section allowing flue gas to come into contact with a basic amine compound absorbent so that the basic amine compound absorbent absorbs CO₂ in the flue gas, the water-washing section allowing the decarbonated flue gas from which CO₂ has been absorbed in the CO₂ absorbing section to come into contact with circulating wash water and to be washed with the wash water so that basic amine compounds entrained in the decarbonated flue gas are reduced in amount; a regenerator releasing the CO₂ from the basic amine compound absorbent containing the CO₂ absorbed into; and a concentrating unit for concentrating the wash water from the water-washing section, the concentrating unit returning condensed water generated during concentration to the water-washing section.

In this CO₂ recovery system, since the condensed water generated when the wash water from the water-washing section is concentrated is returned to the water-washing section, the concentrations of the basic amine compounds in the wash water circulating in the water-washing section are reduced, and the efficiency of washing with the wash water is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be reduced, and the concentrations of the residual basic amine compounds emitted together with the decarbonated flue gas can thereby be further reduced.

Advantageously, in the CO₂ recovery system, the water-washing section includes a plurality of water-washing sections disposed in a path through which the decarbonated flue gas flows upward, and the concentrating unit concentrates part of the wash water discharged from an uppermost one of the plurality of water-washing sections to form a concentrated solution, the concentrated solution being delivered to a lower one of the plurality of water-washing sections, the condensed water being returned to the uppermost water-washing section.

In this CO₂ recovery system, since the condensed water generated when the wash water from the uppermost water-washing section is concentrated is returned to the uppermost water-washing section, the concentrations of the basic amine compounds in the wash water circulating in the uppermost water-washing section are reduced, and the efficiency of washing with the wash water in the uppermost water-washing section is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced.

According to another aspect of the present invention, a CO₂ recovery method includes the steps of: bringing flue gas into contact with a basic amine compound absorbent so that the basic amine compound absorbent absorbs CO₂ contained in the flue gas; washing the decarbonated flue gas in which an amount of CO₂ has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced; releasing CO₂ from the basic amine compound absorbent containing the CO₂ absorbed therein; and concentrating the wash water and delivering condensed water generated during concentration to the step of washing with the wash water.

In this CO₂ recovery method, the decarbonated flue gas is washed with wash water by bringing the decarbonated flue gas into contact with the wash water so that the amounts of basic amine compounds entrained in the decarbonated flue gas are reduced. The wash water used in the step of washing is concentrated, and the condensed water generated during concentration is returned to the step of washing with the wash water. In this manner, the concentrations of the basic amine compounds in the circulating wash water are reduced, and the efficiency of washing with the wash water is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced.

Advantageously, in the CO₂ recovery method, the step of washing the decarbonated flue gas in which an amount of CO₂ has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced is performed at a plurality of stages in a flow path through which the decarbonated flue gas flows upward, and wherein, in the step of concentrating the wash water, the wash water from the step of washing performed at an uppermost stage is concentrated to form a concentrated solution, the concentrated solution being delivered to the step of washing performed at a lower stage in the flow path, the condensed water being returned to the step of washing performed at the uppermost stage.

In this CO₂ recovery method, since the condensed water generated when the wash water is concentrated is delivered to the washing step performed at the uppermost position, the concentrations of the basic amine compounds in the wash water circulated in the washing step performed at the uppermost position are reduced, and the efficiency of washing with the wash water in the washing step performed at the uppermost position is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated flue gas can thereby be further reduced.

Advantageous Effects of Invention

According to the present invention, the concentrations of the residual basic amine compounds emitted together with the decarbonated flue gas can be further reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a CO₂ recovery system according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram of a CO₂ recovery system according to a second embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments. Components in the following embodiments include those which can be easily replaced by persons skilled in the art and also include substantially equivalent components.

A first embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram of a CO₂ recovery system according to the first embodiment.

As shown in FIG. 1, this CO₂ recovery system includes a cooling unit 1, an absorber 2, and a regenerator 3. The cooling unit 1 cools flue gas 101 discharged from an industrial facility (not shown) such as a boiler with cooling water 102. The absorber 2 allows a lean solution 103 a of a basic amine compound absorbent 103, which is an aqueous solution of a basic amine compound that absorbs CO₂, to come into countercurrent contact with the flue gas 101 so that the basic amine compound absorbent 103 absorbs CO₂ in the flue gas 101. The flue gas 101 in which the amount of CO₂ has been reduced is discharged from the absorber 2. The regenerator 3 releases CO₂ from a rich solution 103 b of the basic amine compound absorbent 103 that contains the CO₂ absorbed therein, so that the lean solution 103 a is regenerated.

The flue gas 101 containing CO₂ is increased in pressure by a flow gas blower (not shown) and is delivered inside the cooling unit 1. In the cooling unit 1, the flue gas 101 comes into countercurrent contact with the cooling water 102 and is thereby cooled.

The cooling water 102 is accumulated in the lower portion of the cooling unit 1, and is supplied by a cooling water circulation pump 1 a to the upper portion of the cooling unit 1 through a cooling water tube 1 b disposed outside the cooling unit 1. Then the cooling water 102 flows downward from nozzles 1 c disposed in the upper portion of the cooling unit 1 while the cooling water 102 comes into countercurrent contact with the rising flue gas 101 at the position of a packed bed 1 d disposed between the nozzles 1 c and the lower portion of the cooling unit 1. The cooling water tube 1 b is provided with a cooling unit 1 e. The cooling water 102 is thereby cooled to a temperature lower than the temperature of the flue gas 101, so that part of water in the flue gas 101 is condensed in the cooling unit 1 to form condensed water. The flue gas 101 cooled in the cooling unit 1 is discharged from the cooling unit 1 through a flue gas tube 1 f and is then supplied to the absorber 2.

The absorber 2 includes a CO₂ absorbing section 21 disposed in the lower half thereof, a water-washing section 22 disposed in the upper half, and a concentrating unit 24 disposed outside the absorber 2. In the CO₂ absorbing section 21, the flue gas 101 supplied from the cooling unit 1 comes into countercurrent contact with the lean solution 103 a of the basic amine compound absorbent 103, so that the basic amine compound absorbent 103 absorbs CO₂ in the flue gas 101.

The lean solution 103 a of the basic amine compound absorbent 103 is supplied from the regenerator 3 and flows downward from nozzles 21 a. After coming into countercurrent contact with the rising flue gas 101 at the position of a packed bed 21 b disposed between the nozzles 21 a and the lower portion of the absorber 2, the lean solution 103 a becomes the rich solution 103 b containing CO₂ absorbed therein, and the rich solution 103 b is accumulated in the bottom portion of the absorber 2. The rich solution 103 b of the basic amine compound absorbent 103 accumulated in the bottom portion of the absorber 2 is pumped by a rich-solution discharge pump 21 d disposed in a rich-solution tube 21 c disposed outside the absorber 2 and is then supplied to the regenerator 3. The decarbonated flue gas 101A in which the amount of CO₂ has been reduced flows upward and passes through a demister 21 e, and the basic amine compound absorbent 103 in a mist form that is entrained in the decarbonated flue gas 101A is thereby collected.

The water-washing section 22 allows the decarbonated flue gas 101A in which the amount of CO₂ has been reduced in the CO₂ absorbing section 21 to come into countercurrent contact with wash water 104, so that the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A are reduced through the wash water 104. The decarbonated-deaminated flue gas 101B in which the amounts of the basic amine compounds have been reduced is discharged to the outside of the CO₂ recovery system (the absorber 2).

The wash water 104 flows downward from nozzles 22 a, while the wash water 104 comes into countercurrent contact with the rising decarbonated flue gas 101A at the position of a packed bed 22 b disposed between the nozzles 22 a and a water-washing section water receiver 22 c. Then, the wash water 104 is accumulated in the water-washing section water receiver 22 c. The wash water 104 accumulated in the water-washing section water receiver 22 c is pumped and circulated by a wash water circulation pump 22 e disposed in a wash water tube 22 d disposed outside the absorber 2, while the wash water 104 is cooled in a cooling unit 22 f, and again flows downward from the nozzles 22 a. More specifically, the wash water 104 is circulated and comes into countercurrent contact with the decarbonated flue gas 101A so that the basic amine compounds in the decarbonated flue gas 101A are reduced in amount. After the basic amine compounds in the decarbonated flue gas 101A are reduced in amount through the wash water 104, the resultant decarbonated-deaminated flue gas 101B further flows upward and passes through a demister 22 g, and the wash water 104 in a mist form that is entrained in the decarbonated-deaminated flue gas 101B is thereby collected. The basic amine compounds include, in addition to the basic amine compound used as absorbent, low-molecular weight basic amine compounds generated through decomposition.

In the water-washing section 22, part of condensed water (*1) generated when the CO₂ gas separated from the basic amine compound absorbent 103 in the regenerator 3 is cooled or part of condensed water (*2) generated when the flue gas 101 is cooled in the cooling unit 1 is supplied, as the wash water 104, to the wash water tube 22 d at a position upstream of the cooling unit 22 f. The wash water 104 spilled over the water-washing section water receiver 22 c of the water-washing section 22 is discharged to an overflow tube 22 h disposed outside the absorber 2 and is then supplied to the bottom of the CO₂ absorbing section 21.

The concentrating unit 24 discharges part of the wash water 104 circulating in the water-washing section 22 to the outside of the absorber 2 for concentration. The concentrating unit 24 is connected between a wash water discharge pump 22 e and the cooling unit 22 f disposed in the wash water tube 22 d of the water-washing section 22 through a discharge tube 24 a and is also connected to the water-washing section water receiver 22 c of the water-washing section 22 through a return tube 24 b and to the CO₂ absorbing section 21 (the overflow tube 22 h) through a return tube 24 c. The return tube 24 c may be directly connected to the bottom portion of the CO₂ absorbing section 21.

Preferably, the concentrating unit 24 is a multiple-effect evaporator or a vapor compression condenser. The multiple-effect evaporator includes a plurality of evaporators. The wash water 104 is accumulated in a first one of the evaporators and is then heated and evaporated. The concentrated wash water 104 is supplied to the next evaporator, and the generated vapor is used as the heat source for the next evaporator.

In the vapor compression condenser, vapor generated in an evaporator is pressurized by a compressor to increase the temperature and is used as a heat source for heating. With the vapor compression condenser, the power consumption during concentration can be reduced.

In such a concentrating unit 24, the wash water 104 pumped through the discharge tube 24 a by the wash water discharge pump 22 e is concentrated, and the concentrated wash water 104 is returned to the CO₂ absorbing section 21 through the return tube 24 c. Condensed water generated during concentration is delivered to the water-washing section water receiver 22 c through the return tube 24 b, and the concentrations of basic amines in the wash water 104 can thereby be reduced.

The regenerator 3 includes an absorbent regenerating unit 31 disposed in the lower half thereof. In the absorbent regenerating unit 31, CO₂ is recovered from the rich solution 103 b to regenerate the basic amine compound absorbent 103 as the lean solution 103 a, thereby releasing CO₂ from the basic amine compound absorbent 103 containing the CO₂ absorbed therein.

The rich solution 103 b of the basic amine compound absorbent 103 is supplied from the rich-solution tube 21 c of the CO₂ absorbing section 21 in the absorber 2 and flows downward from nozzles 31 a. Then the rich solution 103 b passes through a lower packed bed 31 b disposed between the nozzles 31 a and the lower portion of the regenerator 3 and is thereby converted to the lean solution 103 a from which substantially the entire amount of CO₂ has been released through endothermic reaction caused by a regenerating heater 31 c connected to the lower portion of the regenerator 3. The resulting lean solution 103 a is accumulated in the bottom portion of the regenerator 3. A sub-regenerator 31 d is connected to the lower portion of the regenerator 3. In the sub-regenerator 31 d, part of the lean solution 103 a is heated. Therefore, degraded products, and the like are concentrated and collected as sludge, and the generated vapor is returned to the lower portion of the regenerator 3. The lean solution 103 a accumulated in the lower portion of the regenerator 3 is pumped by a lean solution discharge pump 31 f disposed in a lean solution tube 31 e and is supplied to the absorber 2. During this process, the lean solution 103 a is heat-exchanged in a rich-lean heat exchanger 4 with the rich solution 103 b supplied to the regenerator 3 through the rich-solution tube 21 c and is cooled in a cooling unit 31 g.

The released CO₂ flows upward in the regenerator 3, passes through an upper packed bed 31 h, and is discharged from the top portion of the regenerator 3. Since the discharged CO₂ contains water, it is cooled in a condenser 32 b disposed in a CO₂ discharge line 32 a. The water contained in the CO₂ is thereby condensed, and the condensed water 106 is separated from CO₂ in a CO₂ separator 32 c. The high-purity CO₂ separated from the condensed water 106 is emitted to the outside of the decarbonation system through a CO₂ emission line 32 d and is used or disposed of in the subsequent process. The condensed water 106 is delivered by a condensed water pump 32 e, and part of the condensed water 106 is supplied to the regenerator 3 from nozzles 32 g disposed in the top portion of the regenerator 3 through a regenerator reflux water line 32 f. The condensed water 106 has a very low amine concentration and therefore can be used as the replenishing water for the water-washing section 22.

As described above, the CO₂ recovery system of the first embodiment includes the absorber 2 and the regenerator 3. The absorber 2 includes the CO₂ absorbing section 21 and the water-washing section 22. The CO₂ absorbing section 21 allows the flue gas 101 containing CO₂ to come into contact with the basic amine compound absorbent 103 so that the basic amine compound absorbent 103 absorbs CO₂ in the flue gas 101. The water-washing section 22 allows the decarbonated flue gas 101A in which the amount of CO₂ has been reduced in the CO₂ absorbing section 21 to come into contact with the circulating wash water 104 and to be washed with the wash water 104 so that the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A are reduced. The regenerator 3 release the CO₂ from the basic amine compound absorbent 103 containing the CO₂ absorbed in. The CO₂ recovery system further includes the concentrating unit 24 for concentrating the wash water 104 discharged from the water-washing section 22. The concentrated solution is delivered to the CO₂ absorbing section 21, and the condensed water generated during concentration is returned to the water-washing section 22.

In this CO₂ recovery system, since the wash water 104 from the water-washing section 22 is concentrated and the condensed water is returned to the water-washing section 22, the concentrations of the basic amine compounds in the wash water 104 circulating in the water-washing section 22 are reduced by a factor of about 10, and the efficiency of washing with the wash water 104 is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas 101A can be reduced by a factor of about 10, and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas 101B can be further reduced.

A CO₂ recovery method of the first embodiment includes the steps of: bringing the flue gas 101 containing CO₂ into contact with the basic amine compound absorbent 103 to reduce the amount of CO₂ contained in the flue gas 101; washing the decarbonated flue gas 101A in which the amount of CO₂ has been reduced with the circulating wash water 104 by bringing the decarbonated flue gas 101A into contact with the wash water 104 so that the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A are reduced; and releasing CO₂ from the basic amine compound absorbent 103 that has absorbed the CO₂. The CO₂ recovery method further includes the step of concentrating the wash water 104 used in the step of washing with the wash water 104 to form a concentrated solution, and delivering the concentrated solution to the CO₂ absorbing step, and returning the condensed water generated during concentration to the step of washing with the wash water 104.

In this CO₂ recovery method, the decarbonated flue gas 101A in which the amount of CO₂ has been reduced is brought into contact with the wash water 104 to reduce the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A, and the wash water 104 used in this step of reducing the amounts of the basic amine compounds is concentrated. The concentrated solution is delivered to the CO₂ absorbing step, and the condensed water is returned to the step of washing with the wash water. In this manner, the concentrations of the basic amine compounds in the wash water 104 are reduced by a factor of about 10, and the efficiency of washing with the wash water 104 is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas 101A can be reduced by a factor of about 10, and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas 101B can be further reduced.

[Second Embodiment]

A second embodiment will be described with reference to the drawing. FIG. 2 is a schematic diagram of a CO₂ recovery system according to embodiment 2.

As shown in FIG. 2, the CO₂ recovery system of the second embodiment includes, in addition to the components in the first embodiment described above, a water-washing section 22 including a plurality of sections disposed in a path through which the flue gas (decarbonated flue gas) 101 flows upward. The concentrating unit 24 is configured such that wash water 104B from an uppermost second water-washing section 22B is concentrated. The concentrated solution is delivered to a lower first water-washing section 22A, and the condensed water generated during concentration is returned to the uppermost second water-washing section 22B. The other components of the CO₂ recovery system of the second embodiment are the same as those of the CO₂ recovery system of the first embodiment. Therefore, the same components are designated by the same reference numerals, and their description will be omitted.

In the present embodiment, the water-washing section 22 includes the first water-washing section 22A and the second water-washing section 22B that are disposed in the path through which the decarbonated flue gas 101A flows upward. The lower first water-washing section 22A allows the decarbonated flue gas 101A in which the amount of CO₂ has been reduced in the CO₂ absorbing section 21 to come into countercurrent contact with wash water 104A, so that the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A are reduced through the wash water 104A.

The wash water 104A flows downward form nozzles 22Aa, while the wash water 104A comes into countercurrent contact with the rising decarbonated flue gas 101A at the position of a packed bed 22Ab disposed between the nozzles 22Aa and a water-washing section water receiver 22Ac. Then, the wash water 104A is accumulated in the water-washing section water receiver 22Ac. The wash water 104A accumulated in the water-washing section water receiver 22Ac is pumped and circulated by a wash water discharge pump 22Ae disposed in a wash water tube 22Ad disposed outside the absorber 2, while the wash water 104A cooled in a cooling unit 22Af, and again flows downward from the nozzles 22Aa. More specifically, the wash water 104A is circulated and comes into countercurrent contact with the decarbonated flue gas 101A so that the basic amine compounds in the decarbonated flue gas 101A are reduced in amount. After the basic amine compounds are reduced in amount through the wash water 104A, the resultant decarbonated flue gas 101A further flows upward and passes through a demister 22Ag, and the wash water 104A in a mist form that is entrained in the decarbonated flue gas 101A is thereby collected.

The upper second water-washing section 22B allows the rising decarbonated flue gas 101A that has passed through the first water-washing section 22A to come into countercurrent contact with wash water 104B so that the basic amine compounds entrained in the decarbonated flue gas 101A are further reduced in amount through the wash water 104B. The resultant decarbonated-deaminated flue gas 101B in which the amounts of the basic amine compounds has been reduced is discharged to the outside of the CO₂ recovery system (the absorber 2).

The wash water 104B flows downward from nozzles 22Ba, while the wash water 104B comes into countercurrent contact with the rising decarbonated flue gas 101A at the position of a packed bed 22Bb disposed between the nozzles 22Ba and a water-washing section water receiver 22Bc. Then, the wash water 104B is accumulated in the water-washing section water receiver 22Bc. The wash water 104B accumulated in the water-washing section water receiver 22Bc is pumped and circulated by a wash water circulation pump 22Be disposed in a wash water tube 22Bd disposed outside the absorber 2, while the wash water 104B is cooled by a cooling unit 22Bf, and again flows downward from the nozzles 22Ba. More specifically, the wash water 104B is circulated and comes into countercurrent contact with the decarbonated flue gas 101A so that the basic amine compounds in the decarbonated flue gas 101A are reduced in amount. After the basic amine compounds are reduced in amount through the wash water 104B, the resultant decarbonated-deaminated flue gas 101B further flows upward and passes through a demister 22Bg, and the wash water 104B in a mist form that is entrained in the decarbonated-deaminated flue gas 101B is thereby collected.

In the second water-washing section 22B, part of condensed water (*1) generated when the CO₂ gas separated from the basic amine compound absorbent 103 in the regenerator 3 is cooled or part of condensed water (*2) generated when the flue gas 101 is cooled in the cooling unit 1 is supplied, as the wash water 104B, to the wash water tube 22Bd at a position upstream of the cooling unit 22Bf. The wash water 104B spilled over the water-washing section water receiver 22Bc of the second water-washing section 22B is discharged to an overflow tube 22Bh disposed outside the absorber 2 and then supplied to the water-washing section water receiver 22Ac of the first water-washing section 22A. The wash water 104A spilled over the water-washing section water receiver 22Ac of the first water-washing section 22A is discharged to an overflow tube 22Ah disposed outside the absorber 2 and then supplied to the bottom of the CO₂ absorbing section 21. The water-washing section 22 may include two or more sections.

In the concentrating unit 24, part of the wash water 104B circulating in the uppermost second water-washing section 22B is discharged from the absorber 2 and then concentrated. The concentrating unit 24 is connected between the wash water circulation pump 22Be and the cooling unit 22Bf disposed in the wash water tube 22Bd of the second water-washing section 22B through the discharge tube 24 a and is also connected to the water-washing section water receiver 22Bc of the second water-washing section 22B through the return tube 24 and to the water-washing section water receiver 22Ac of the first water-washing section 22A through the return tube 24 c.

Preferably, the concentrating unit 24 is a multiple-effect evaporator or a vapor compression condenser. The multiple-effect evaporator includes a plurality of evaporators. The wash water 104B is accumulated in a first one of the evaporators and is then heated and evaporated. The concentrated wash water 104B is supplied to the next evaporator, and the generated vapor is used as the heat source for the next evaporator.

In the vapor compression condenser, vapor generated in an evaporator is pressurized by a compressor to increase the temperature and is used as a heat source for heating. With the vapor compression condenser, the power consumption during concentration can be reduced.

In such a concentrating unit 24, the wash water 104B supplied through the discharge tube 24 a by the wash water discharge pump 22Be is concentrated. The concentrated solution is delivered to the first water-washing section 22A through the return tube 24 c, and the condensed water is returned to the second water-washing section 22B through the return tube 24 b.

As described above, in the CO₂ recovery system of the second embodiment, the water-washing section 22 includes a plurality of sections disposed in the path through which the decarbonated flue gas 101A flows upward. In the concentrating unit 24, part of the wash water 104B discharged from the uppermost second water-washing section 22B is concentrated. The concentrated solution is delivered through the return tube 24 c to the first water-washing section 22A disposed on the upstream side of the flow of the decarbonated flue gas 101A, and the condensed water is returned to the uppermost second water-washing section 22B through the return tube 24 b.

In this CO₂ recovery system, the wash water 104B used in the uppermost second water-washing section 22B is concentrated, and the condensed water is returned to the second water-washing section 22B through the return tube 24 b. In this manner, the concentrations of the basic amine compounds in the wash water 104B circulating in the second water-washing section 22B are reduced, and the efficiency of washing in the second water-washing section 22B is thereby improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas 101A can be further reduced, and the concentrations of the residual basic amine compounds emitted together with the decarbonated-deaminated flue gas 101B can be further reduced. Since the concentrated solution is delivered to the first water-washing section 22A through the return tube 24 c, the flow rate of water discharged from the first water-washing section 22A is increased. Therefore, the concentrations of the basic amine compounds in the circulation water in the first water-washing section 22A are reduced, and the concentrations of the basic amine compounds in the decarbonated flue gas supplied to the second water-washing section 22B are reduced. The concentrated solution may be returned to the CO₂ absorbing section 21. This is also effective to reduce the concentrations of the basic amine compounds in the decarbonated flue gas.

In a CO₂ recovery method of the second embodiment, the step of washing the decarbonated flue gas 101A in which the amount of CO₂ has been reduced with the wash water 104 by bringing the decarbonated flue gas 101A into contact with the wash water 104 so that the amounts of the basic amine compounds entrained in the decarbonated flue gas 101A are reduced is performed at a plurality of positions in the path through which the decarbonated flue gas 101A flows upward. In the step of concentrating the wash water 104, the wash water 104B from the water washing step performed at the uppermost position (the second water-washing section 22B) is concentrated. The concentrated solution is delivered to the water washing step performed at a lower position (the first water-washing section 22A disposed on the upstream side of the flow of the decarbonated flue gas 101A), and the condensed water is returned to the water washing step performed at the uppermost position (the second water-washing section 22B). The recovery method of the present embodiment includes this step.

In this CO₂ recovery method, since the condensed water is returned to the water washing step performed at the uppermost position, the concentrations of the basic amine compounds in the wash water 104 circulated in the water washing step performed at the uppermost position are reduced, and the washing efficiency of the water washing step performed at the uppermost position is improved. Therefore, the concentrations of the basic amine compounds in the decarbonated flue gas 101A can be further reduced, and the concentrations of the residual basic amine compounds discharged together with the decarbonated-deaminated flue gas 101B can be further reduced.

INDUSTRIAL APPLICABILITY

As described above, the CO₂ recovery system and method according to the present invention are suitable for further reducing the concentrations of the basic amine compounds remaining in the decarbonated flue gas.

REFERENCE SIGNS LIST

-   1 cooling unit. -   2 absorber. -   21 CO₂ absorbing section. -   22 water-washing section. -   22 a nozzle. -   22 b packed bed. -   22 c water-washing section water receiver. -   22 d wash water tube. -   22 e wash water discharge pump. -   22 f cooling unit. -   22 g demister. -   22 h overflow tube. -   22A first water-washing section. -   22Aa nozzle. -   22Ab packed bed. -   22Ac water-washing section water receiver. -   22Ad wash water tube. -   22Ae wash water discharge pump. -   22Af cooling unit. -   22Ag demister. -   22Ah overflow tube. -   22B second water-washing section. -   22Ba nozzle. -   22Bb packed bed. -   22Bc water-washing section water receiver. -   22Bd wash water tube. -   22Be wash water discharge pump. -   22Bf cooling unit. -   22Bg demister. -   22Bb overflow tube. -   24 concentrating unit. -   24 a discharge tube. -   24 b return tube. -   24 c return tube. -   3 regenerator. -   4 rich-lean heat exchanger. -   101 flue gas. -   101A decarbonated flue gas. -   101B decarbonated-deaminated flue gas. -   102 cooling water. -   103 basic amine compound absorbent. -   103 a lean solution. -   103 b rich solution. -   104 (104A, 104B) wash water. 

The invention claimed is:
 1. A CO₂ recovery method, comprising the steps of: bringing flue gas into contact with a basic amine compound absorbent so that the basic amine compound absorbent absorbs CO₂ contained in the flue gas; washing the decarbonated flue gas in which an amount of CO₂ has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced; releasing CO₂ from the basic amine compound absorbent containing the CO₂ absorbed therein; and concentrating the wash water and delivering condensed water generated during concentration to the step of washing with the wash water; wherein the step of washing the decarbonated flue gas in which an amount of CO₂ has been reduced with circulating wash water by bringing the decarbonated flue gas into contact with the wash water so that amounts of basic amine compounds entrained in the decarbonated flue gas are reduced is performed at a plurality of stages in a flow path through which the decarbonated flue gas flows upward, and wherein, in the step of concentrating the wash water, the wash water from the step of washing performed at an uppermost stage is concentrated to form a concentrated solution, the concentrated solution being delivered to the step of washing performed at a lower stage in the flow path, the condensed water being returned to the step of washing performed at the uppermost stage. 